This invention relates to a heating device for heat-treating an object to be heated, and in particular to a heating device effective to heat and fix unfixed images such as toner images.
2. Description of the Prior Art
In heating devices using heat, it has heretofore been difficult to achieve uniform heating and power saving. Particularly in heat roller fixing devices which are typical fixing devices, there has been a problem that the heat from a heating source provided in the image forming apparatus or the fixing device is lost without being transmitted to the surface of the heat roller for use for fixation. As a result, the heat loss is great at the end portions of the heat roller and the fixing efficiency is greatly reduced.
Also, it has usually been practised to provide a metal plate of a thickness of several millimeters or more or to an insulating member of a thickness of several centimeters around the heating device, as disclosed in U.S. Pat. No. 3,998,584.
In the prior art, design has been made such that the heating distribution of the heating source is stronger in the end areas of the roller to compensate for any temperature drop at the end portions of the heat roller or at the end portions of the pressure roller or the like urged against the heat roller. This has led to increased power consumption in the fixing device, which in turn has led to the necessity of limiting the power supply to other stations in the apparatus or to the unavoidable complication of the temperature control in the fixing device.
On the other hand, even with such approach, the amount of heat lost from the end portions of the heat roller is increased and this induces temperature rise in the interior of the image forming apparatus, which in turn may induce toner blocking in the cleaner or the developing device or thermal deformation of the plastic molded parts in the apparatus.
As another method of solution, it has already been practised to provide an insulating sleeve between the heat roller and the bearing member holding the heat roller or provide a plain bearing formed of resin to decrease the abovedescribed heat loss, as disclosed in U.S. Pat. No. 3,945,726. However, this method cannot after all provide a satisfactory solution but still induces great heat loss which causes a temperature drop in the end portions of the roller.
In the above-described countermeasure wherein it is usually practised to intensify the heating distribution at the end portions of the roller, the power consumption as a whole does not differ from that in the conventional devices.
The above-described method induces not only waste of power but also temperature rise in other portions of the apparatus body, which in turn may result in various problems.
Another countermeasure is proposed in Japanese Laid-open Utility Model Application No. 145061/1981. This method basically comprises interposing an insulating material between the shaft of the heating roller and the gear, and intends to prevent heat dissipation from the end portions of the roller, together with the aforementioned insulating sleeve. However, the heater provided in this heating roller exhibits a large heating compatibility at the end portions of the roller whereat the gear and bearing are mounted and therefore, it expedites deterioration of the insulating material. Also, the thermal imbalance on the surface of the heating roller is increased to cause unsatisfactory fixation to result from heat deficiency in the central portion of the roller, and this complicates the temperature control on the surface of the roller.
Thus, in the devices of the prior art, effective utilization of heat and power could not be achieved. Also, in the devices of the prior art, when continuous fixation of several tens of copies was carried out, the thermal non-uniformity on the surface of the heating roller was increased to prevent sufficient fixation from being accomplished and thus, the apparatus was constrained to be stopped from operation.
In the recent heating devices and image forming apparatuses equipped with the same, power saving is desired for the purpose of effective utilization of power. Further, the advent of a heating device capable of effecting high-speed heat treatment efficiently, uniformly and stably for the object to be heated and the support member therefor is desired.